This invention relates to axial flow fans, for example, fans designed to move a fluid such as air through a heat exchanger such as an air conditioning condenser.
When selecting an axial fan for a particular application, one of the parameters to be chosen is the non dimensional loading. Non-dimensional loading is the ratio of the change of pressure across the fan to the product the density of the fluid moved by the fan and the square of the speed of the tips of the fan blades. Since non-dimensional loading is inversely proportional to the square of the tip speed, heavily loaded fans will generally have lower tip speeds, assuming the pressure drop and fluid density are relatively constant. There are several advantages to operating a fan at lower speeds (i.e., with higher non-dimensional loading) including reduced noise and vibration levels and reduced centrifugal forces acting on the fan. In addition, limits on the diameter and the capability of a particular engine or electric motor may require that the non-dimensional loading be high.
When a heavily loaded fan is used in a given application, e.g., moving air, large tangential, or swirl velocities are imparted to the air as it moves through the fan. These swirl velocities cause centrifugal forces to act on the air as it leaves the fan. In the absence of other forces acting on the air, the air will move radially under the action of these centrifugal forces and the jet of air leaving the fan will therefore not be of constant radius, but will expand downstream of the fan.
An axial fan that is designed to push air through a compact heat exchanger, such as an air-conditioning condenser or automotive radiator, is positioned in a shroud which directs all of the air through the core of the heat exchanger. Typically, this shroud is only slightly larder than the fan itself, but is rectangular in shape rather than circular. When using a heavily loaded fan, an expanding jet of air, as discussed above, will leave the fan and impinge on the sides of the shroud rather than the core. The sides of the shroud must then turn the flow, and force the air through the edges of the core.